This invention relates to an idling speed feedback control method for internal combustion engines, and more particularly to a method of this kind which can prevent engine stall at transition of the engine operation from a decelerating condition with the throttle valve fully closed to the idling speed feedback controlling condition.
An internal combustion engine in general can easily stall due to a drop in the engine speed when the engine is operated in an idling condition while engine coolant temperature is low or when the engine is heavily loaded with electrical loads by head lamps, electric blower, air-conditioner, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling speed feedback control method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98628, which comprises setting a desired idling speed in dependence upon load on the engine, detecting the difference between the actual engine speed and the desired idling speed, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine speed to the desired idling speed.
According to this proposed method, if the above idling speed feedback control is carried out when the engine is decelerating toward the idling region with the throttle valve fully closed, the engine speed can abruptly decreases, depending upon the engine temperature and electrical load such as the air-conditioner. Then, even if idling speed feedback control, hereinafter referred to, is started immediately following the abrupt decrease in engine speed, it cannot immediately follow the abrupt speed decrease to supply a required quantity of supplementary air to the engine, often resulting in engine stall.
To avoid this disadvantage, an idling speed feedback control method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98629. According to this proposed method, in transition from a decelerating condition to an idling speed feedback controlling condition, the quantity of supplementary air is controlled in decelerating mode wherein the quantity of supplementary air required for maintaining the engine speed at the desired idling speed is estimated in advance of the completion of the transition of the engine operation and the estimated quantity of supplementary air is previously supplied to the engine before the idling speed feedback control is started, to thereby ensure smooth transition to the idling operation.
However, even if the above decelerating mode control of supplementary air quantity is carried out in advance of the completion of the transition of the engine operation to the idling speed feedback controlling condition, for instance, when the clutch is disengaged to disconnect the engine and the driving wheels, or when the engine is racing, the engine speed can suddenly drop far below the desired idling speed (i.e. engine speed overshoot), resulting in a delay in controlling the engine speed to the desired idling speed, even if supplementary air has been previously supplied by the above proposed method before the feedback control starts.
In order to avoid this disadvantage, the quantity of supplementary air previously supplied before the start of feedback control is increased by a predetermined quantity, then, the engine speed will not promptly reach the idling speed at slow deceleration of the engine (i.e. the engine speed undershoots). Thus this method also cannot solve the problem of delay in controlling the engine speed to the desired idling speed.